1. Field of the Invention
The present invention relates to a method of preventing corrosion of metal surfaces. More particularly, the present invention provides a method of preventing corrosion of a metal surface which comprises applying a solution containing one or more bis-functional polysulfur silanes to the metal surface. The method is particularly useful for treating surfaces of zinc, copper, aluminum, and alloys of the foregoing metals (such as brass and bronze).
2. Description of Related Art
Most metals are susceptible to varying degrees and types of corrosion which will significantly affect the quality of such metals, as well as that of the products produced therefrom. Although many forms of corrosion can sometimes be prevented, such steps are costly and may further diminish the utility of the final product. In addition, when polymer coatings such as paints, adhesives, or rubbers are applied to the metal, corrosion of the base metal material may cause a loss of adhesion between the polymer coating and the base metal.
Prior art techniques for improving corrosion resistance of metals, particularly metal sheet, include passivating the surface by means of a heavy chromate treatment. Such treatment methods are undesirable, however, because the chromate ion is highly toxic, carcinogenic and environmentally undesirable. It is also known to employ a phosphate conversion coating in conjunction with a chromate rinse in order to improve paint adherence and provide corrosion protection. It is believed that the chromate rinse covers the pores in the phosphate coating, thereby improving the corrosion resistance and adhesion performance. Once again, however, it is highly desirable to eliminate the use of chromate altogether. Unfortunately, the phosphate conversion coating is generally not effective without the chromate rinse.
Recently, various techniques for eliminating the use of chromate have been proposed. These include coating the metal with an inorganic silicate followed by treating the silicate coating with an organofunctional silane (U.S. Pat. No. 5,108,793). U.S. Pat. No. 5,292,549 teaches the rinsing of metal sheet with a solution containing an organofunctional silane and a crosslinking agent in order to provide temporary corrosion protection. The crosslinking agent crosslinks the organofunctional silane to form a denser siloxane film. One significant drawback of the methods of this patent, however, is that the organofunctional silane will not bond well to the metal surface, and thus the coating of U.S. Pat. No. 5,292,549 may be easily rinsed off. Various other techniques for preventing the corrosion of metal sheets have also been proposed. Many of these proposed techniques, however, are ineffective, or require time-consuming, energy-inefficient, multi-step processes.
Further complicating the task of preventing corrosion of metals is the fact that corrosion can occur by a number of different mechanisms, depending in large part upon the particular metal in question. Brass, for example, is very sensitive to corrosion in aqueous environments (particularly uniform corrosion), dezincification (especially in acid-chloride containing solutions), and stress corrosion cracking (particularly in the presence of ammonia and amines). Copper, and copper alloys (including brass) will tarnish readily in air and in sulfur-containing environments. Zinc, and zinc alloys, on the other hand, are particularly susceptible to the formation of "white rust" under humid conditions. Unfortunately, many of the prior art treatment methods for preventing corrosion are less effective on zinc, zinc alloys, copper, and copper alloys, especially brass and bronze, or are only effective for certain types of corrosion.
Thus, there is a need for a simple, low-cost technique for preventing corrosion of metal surfaces, particularly zinc, zinc alloys, aluminum, aluminum alloys, copper, and copper alloys (especially brass and bronze).